Introduction to Metal Cutting

Introduction to

Metal Cutting.

CLASSIFICATION OF MANUFACTURING PROCESSES

• Primary shaping processes(Solidification Processes)

• Secondary machining processes(Material Removal Processes)

• Metal forming processes (Deformation Processes)

• Joining processes(Assembly Operations) • Surface finishing processes • Processes effecting change in properties.

CLASSIFICATION OF MANUFACTURING PROCESSES

• Primary shaping processes(Solidification Processes)

• Secondary machining processes(Material Removal Processes)

• Metal forming processes (Deformation Processes)

• Joining processes(Assembly Operations) • Surface finishing processes • Processes effecting change in properties.

Secondary machining processes Material Removal Processes

• Excess material removed from the starting workpiece so

what remains is the desired geometry• Examples: machining such as turning, drilling, and milling; also grinding and nontraditional machining processes

Jobs undergoing these operations are the roughly finished products received through primary shaping processes. The process of removing the undesired or unwanted material from the workpiece or job or component to produce a required shape using a cutting tool is known as machining

• Variety of work materials can be machined– Most frequently applied to metals

• Variety of part shapes and special geometry features possible, such as:– Screw threads– Accurate round holes– Very straight edges and surfaces

• Good dimensional accuracy and surface finish

Why Machining is Important

• Various machining processes are (1) Turning, (2) Threading, (3) Knurling,

(4) Milling, (5) Drilling, (6) Boring, (7) Planning, (8) Shaping, (9) Slotting, (10) Sawing, (11) Broaching, (12) Hobbing, (13) Grinding, (14) Gear cutting, (15) Thread cutting and

(16) Unconventional machining processes using ECM, LBM, AJM, USM setups etc.

Machining Processes

Disadvantages of Machining

• Wasteful of material– Chips generated in machining are wasted material,

at least in the unit operation

• Time consuming– A machining operation generally takes more time

to shape a given part than alternative shaping processes, such as casting, powder metallurgy, or forming

Basic Mechanics of Metal Cutting• Metal ahead of the cutting tool is compressed.

This results in the deformation or elongation of the crystal structure—resulting in a shearing of the metal. As the process continues, the metal above the cutting edge is forced along the “chip-tool” interference zone and is moved away form the work.

Modeling: Mechanism of cutting

Chip

Tool

Chip forms byshear in this regiond

epth

of c

ut

Friction betweentool, chip in thisregion

Chip

Tool

Chip forms byshear in this regiond

epth

of c

ut

Friction betweentool, chip in thisregion

Old model: crack propagation Current model: shear

∅

(∅-α)

Basic Mechanics of Metal Cutting

Machining Terminology

Machining Terminology

Orthogonal and Oblique CuttingThe two basic methods of metal cutting using a single point tool are the orthogonal (2 D) and oblique (3D). Orthogonal cutting takes place when the cutting face of the tool is 90 degree to the line of action of the tool. If the cutting face is inclined at an angle less than 90 degree to the line of action of the tool, the cutting action is known as oblique.

Orthogonal Cutting: The cutting edge of the tool

remains normal to the direction of tool feed or work feed.

The direction of the chip flow velocity is normal to the cutting edge of the tool.

Here only two components of forces are acting: Cutting Force and Thrust Force. So the metal cutting may be considered as a two dimensional cutting.

Oblique Cutting:• The cutting edge of the tool remains

inclined at an acute angle to the direction of tool feed or work feed.

• The direction of the chip flow velocity is at an angle with the normal to the cutting edge of the tool. The angle is known as chip flow angle.

• Here three components of forces are acting: Cutting Force, Radial force and Thrust Force or feed force. So the metal cutting may be considered as a three dimensional cutting.

The cutting edge being oblique, the shear force acts on a larger area and thus tool life is increased.

FeedTool

Work

Oblique cutting

FeedTool

Work

Orthogonal cutting

Orthogonal Vs Oblique Cutting

Tool Geometry

• The geometry of a cutting tool is determined by factors like:– Properties of the tool material– Properties of the workpiece– Type of cut

Tool Geometry

• The most important geometry’s to consider on a cutting tool are – Rake Angles– Relief Angles– Cutting Edge Angles– Nose radius

Tool Angle Application

• Factors to consider for tool angles– The hardness of the metal– Type of cutting operation– Material and shape of the cutting tool– The strength of the cutting edge

Right hand single point cutting tool

FIGURE : (a) Schematic illustration of a right-hand cutting tool. Although these tools have traditionally been produced from solid tool-steel bars, they have been largely replaced by carbide or other inserts of various shapes and sizes, as shown in (b).

Single Point Cutting Tool Geometry

Side rake angle (αs)

End relief angle (ERA)

Nose Radius (NR)

End cutting edge angle (ECEA)

Side cutting edge angle (SCEA)

Side ViewFront View

Top View

Lip angle

Back rake angle (αb)

Side relief angle (SRA)

Geometry of positive rake single point cutting tool

Single Point Cutting Tool GeometryGeometry of negative rake single point cutting tool

Side rake angle (αs)

End relief angle (ERA)

Nose Radius (NR)

End cutting edge angle (ECEA)

Side cutting edge angle (SCEA)

Side ViewFront View

Top View

Lip angle

Back rake angle (αb)

Side relief angle (SRA)

Cutting edge Angles

• Side cutting Edge angle(SCEA)-Cs– Also known as lead angle– Angle between side cutting edge & side of the tool shank– Complementary angle to this is called ‘Approach angle’– Prevents interference as the tool enters the work material– The tip of the tool is protected at the start of the cut– Affect tool life & surface finish– Higher value will have more of its length in action for a given depth of cut

and edge last longer– Chip produced will be thinner and wider– However ,higher values have greater component of forces tending to

separate the work and tool promoting chatter.– 15° to 30°for general machining– Low value for casting and forging having hard and scaly skin

• End cutting Edge angle(ECEA)-Ce– Angle between the end cutting edge & line normal to the tool shank– Provides clearance or relief to the trailing end of the cutting edge to

prevent rubbing or drag between machined surface & trailing part of the cutting edge( non-cutting).

– Only a small angle is sufficient 8° to15° degree– Part off and necking tool often have no end cutting edge angle

Cutting edge Angles

Relief angles

• Side Relief angle(SRF)-θs

– Angle between the portion of the side flank immediately below the side cutting edge and a line perpendicular to the base of the tool

– Measured at right angle to the side flank

• End Relief angle(ER)-θe

– Angle between face of the tool and a line parellel to the base of of the tool

– Measured at right angle to the side flank

Need for Relief angle

• Provided so that flank of the tool clears the work piece surface and there is no rubbing action between the two.

• Ranges from 5° to 15°• Smaller angle are necessary to provide strength to the cutting

edge when machining hard material• Increased value of relief angle penetrate & cut the work piece

material more efficiently and thus reduces the cutting force.• Too large angles weaken the cutting edge & there is less mass

to absorb and conduct the heat away from the cutting edge.

Rake Angles• Back Rake angle (BR)-αb

– Angle between face of the tool and a line parallel to the base of the tool– Measured in a plane perpendicular through the side cutting edge.– Angle is +ve if side cutting edge slopes downward from the point

towards the shank– Angle is –ve when it is reverse i.e. slopes towards the point from shank

• Side Rake angle (SR)- αs

– Angle between face of the tool and a line parallel to the base of the tool– Measured in a plane perpendicular to the base & the side cutting edge.– Angle give the slope of face of the tool from side cutting edge– Angle is -ve if slopes towards the side cutting edge– Angle is +ve when it slopes away from the cutting edge

Importance of Rake Angles

• The top face of the tool over which the chip flows is known as the rake face• Cutting angle and angle of shear are affected by the values for rake angles• Larger the rake angle smaller the cutting angle (and larger the shear angle)

resulting in lower cutting force and power.• However, since increase in rake angles decreases the cutting angle, this

leaves less metal at the point of the tool to support cutting edge and in conducting the heat away.

• Small rake angle for cutting hard material and larger value for softer material.

Importance of Rake Angles

• Use of negative rake angle started with use of carbide tools• Carbide being brittle lacks shock resistance and will fail if

positive rake angle is used with it.• Using negative rake angles directs the force back into the

body of the tool away from the cutting edge giving protection to the cutting edge.

• Use of negative rake angles increases the cutting force but at higher cutting speed at which carbide cutting tools are generally used, this increase in force is less than at normal cutting speeds.

Recommendations w.r.t Rake Angle• Positive rake angles

– When machining low strength ferrous & nonferrous materials and work hardening materials

– When using a low power machine– Long shafts and smaller diameter– When setup lack rigidity– When cutting at low cutting speed

• Negative rake angles– When machining high strength alloy– When there is heavy impact loads such as interrupted

machining.– For rigid setups and when cutting at high cutting speed

The rake angle for a tool depends on the following factors:• Type of material being cut: A harder material like cast iron may be

machined by smaller rake angle than that required by soft material like mid steel or aluminum.

• Type of tool material: Tool material like cemented carbide permits turning at very high speed. At high speeds rake angle has little influence on cutting pressure. Under such condition the rake angle can minimum or even negative rake angle is provided to increase the tool strength.

• Depth of cut: In rough turning, high depth of cut is given to remove maximum amount of material. This means that the tool has to withstand severe cutting pressure. So the rake angle should be decreased to increase the lip angle that provides the strength to the cutting edge.

• Rigidity of the tool holder and machine: An improperly supported tool on old or worn out machine cannot take up high cutting pressure. So while machining under the above condition, the tool used should have larger rake angle.

• The nose of a tool is slightly rounded in all turning tools.

• The function of nose radius is as follows:

• Greater nose radius clears up the feed marks caused by the previous shearing action and provides better surface finish.

• All finish turning tool have greater nose radius than rough turning tools.

• It increases the strength of the cutting edge, tends to minimize the wear taking place in a sharp pointed tool with consequent increase in tool life.

• Accumulation heat is less than that in a pointed tool which permits higher cutting speeds.

Nose radius

Tool signatureIt is the system of designating the principal angles of a single point cutting tool. The signature is the sequence of numbers listing the various angles, in degrees, and the size of the nose radius. There are several systems available like American standard system (ASA), Orthogonal rake system (ORS), Normal rake system (NRS), and Maximum rake system (MRS).The system most commonly used is American Standard Association (ASA), which is:Back rake angle, Side rake angle, End relief angle, Side relief angle, End cutting Edge angle, Side cutting Edge angle and Nose radius.

For example a tool may designated in the following sequence :: 8-14-6-6-6-15-11. Bake rake angle is 82. Side rake angle is 143. End relief angle is 64. Side relief angle is 65. End cutting Edge angle is 66. Side cutting Edge angle is 157. Nose radius is 1 mm

Tool signature

(a) Designations and symbols for a right-hand cutting tool; solid high-speed-steel tools have a similar designation.

Designations for a Right-Handed Cutting Tool

(b) Square insert in a right-hand toolholder for a turning operation.

Chip Formations

• During the machining process (3) basic types of chips are formed: – Discontinuous– Continuous– Continuous with a built-up edge (BUE)

Discontinuous

• Typically associated with brittle metals like –Cast Iron

• As tool contacts work, some compression takes place

• As the chip starts up the chip-tool interference zone, increased stress occurs until the metal reaches a saturation point and fractures off the workpiece.

Discontinuous• Conditions which favor

this type of chip – Brittle work material– Small rake angles on

cutting tools– Coarse machining feeds– Low cutting speeds– Major disadvantage—

could result in poor surface finish

Continuous

• Continuous “ribbon” of metal that flows up the chip/tool zone.

• Usually considered the ideal condition for efficient cutting action.

Continuous

• Conditions which favor this type of chip: – Ductile work– Fine feeds– Sharp cutting tools– Larger rake angles– High cutting speeds– Proper coolants

Continuous with a built-up edge(BUE)

• Same process as continuous, but as the metal begins to flow up the chip-tool zone, small particles of the metal begin to adhere or weld themselves to the edge of the cutting tool. As the particles continue to weld to the tool it effects the cutting action of the tool.

Continuous with a built-up edge(BUE)

• This type of chip is common in softer non-ferrous metals and low carbon steels.

• Problems– Welded edges break off and

can become embedded in workpiece

– Decreases tool life– Can result in poor surface

finishes

Types of Chip in Machining

Factors Influencing the Chip Formation Process